There is a well-known mythabout the hemispheres that grew from "split-brain"
research in the 1960s. In a drastic treatment for epilepsy, surgeons operated a number of patients by cutting the corpus
callosum, the thick bundle of nerve fibres that forms the main
connection between the cerebral hemispheres.

The surgery revealed as described "two spheres of consciousness" locked in the one
head, the left-hand side having speech and a rational, intellectual
style, while the right was inarticulate, but blessed with special
spatial abilities.

Manytheorieshave grown up around these hemispheres such as two personalities in one head, with:

To most
neuroscientists, these notions are seen as simplistic, so there was general satisfaction when, a
couple of years ago. A simple brain scanner test appeared to reveal the
true story about one of neurology's greatest puzzles.

Exactly what is
the difference between the two sides of the human brain?

Clinical neurologists Gereon Fink of
the University of DÃ¼sseldorf in Germany and John Marshall from the
Radcliffe Infirmary in Oxford, pursued the idea that the
difference between the two hemispheres lay in their style of working:

Left:

Focuses on detail.

Mental skills that need us to act in a
series of discrete steps or fix on a particular fragment of what we
perceive, skills such as recognising a friend's face in a crowd or
"lining up" words to make a sentence.

Right:

Concentrates on the broad, background picture.

Panoramic focus that made it good at
seeing general connections; this hemisphere was best able to represent
the relative position of objects in space and to handle the emotional
and metaphorical aspects of speech.

There is a similar balance structure in auditory processing between 'Global' and 'Detailed'. The main difference between Left Hemisphere (LH) and Right Hemisphere (RH) can be found at the micro-anatomical level. There are numerous asymmetries that inﬂuence how neurons spread information. At the cellular level, pyramidal cell dendrites branch further from the soma and ultimately into more branches with more dendritic spines, in the RH than in the LH.

Such circuitry favors more input from relatively distant sources in the RH, and from close sources in the LH. Thus, cortical mini-columns, macro-columns, and functional areas are more highly overlapping and more densely interconnected in the RH than in the LH.

One side of the brain thought and saw in wide-angle while the other zoomed in on the detail. To test this idea, the imaging laboratory at
London's Institute of Neurology, scanned the brains of people who
were looking at a series of images called "letter navons" (image left) and asked their
subjects what they saw: F: Local element - S: Global image

Results:

Concentrating on the small letters (F) fired areas on the left side.

Mentally stepping back, to take in the overall shape (S) fired areas on the right side.

Fork or hat?

In a test split-brain patients matched household objects, when seeing "a cake on a plate".

Left connected to a picture of a fork and spoon -> function.

Right connected to a picture of a broad-brimmed hat -> appearance.

This
evidence supports the idea of a highly modular brain in
which thinking in logical categories is a strictly left
hemisphere function while mental imagery and spatial awareness are
handled on the right.

But, says Joseph Hellige, a psychologist at the
University of Southern California, this picture changed dramatically as
soon as brain-scanning experiments began to show that both sides of the
brain played an active role in such processes.

Processing styles seem to distinguished the two halves. Under the scanner, language and space turned out to be represented on both sides of the brain:

Speech

Space

Left

Grammar and word production.

Left

Objects at particular locations.

Right

Intonation and emphasis.

Right

General sense of space.

With all this evidence, researchers have come to see the distinction
between the two hemispheres as a subtle one of processing style, with
every mental faculty shared across the brain, and each side
contributing in a complementary, not exclusive, fashion. A smart brain
became one that simultaneously grasped both the foreground and the
background of the moment.

Next they had the problem to work
out exactly how the brain manages to produce these two contrasting
styles. According to Hellige, he and many other researchers originally
looked for the explanation in a simple wiring difference within the
brain.

The theory held that, there are different neuron connections:

Left: make
sparser, short-range connections with their neighbours.

Right: more richly and widely connected.

The result
would be that the representation of sensations, memories and even motor
plans would be confined to smallish, discrete areas in the left
hemisphere, while exactly the same input to a corresponding area of the
right side would form a sprawling, even impressionistic, pattern of
activity.

Supporters of this idea argued that these structural
differences would explain why:

left-brain language areas are so good at
precise representation of words and word sequences.

Right
brain seems to supply a wider sense of context and meaning.

A striking
finding from some people who suffer right-brain strokes is that they
can understand the literal meaning of sentences, their left brain can
still decode the words, but they can no longer get jokes or allusions.
Asked to explain even a common proverb, such as "a stitch in time saves
nine", they can only say it must have something to do with sewing. An
intact right brain is needed to make the more playful connections.

Even
though this theory has no anatomical backing (just try counting neural
connections under a microscope), computer simulations made it seem a
decent enough hypothesis. For example, researchers including Robert
Jacobs at the University of Rochester, New York, showed that varying
the richness and distance of interconnections between neurons in an
artificial neural network changes the network's performance. It can be
made good at recognising either specific shapes or at grouping shapes
generally.

But wiring differences are not the only contender to
account for the origin of the brain's hemispheric bias.

One of the main
reasons why Fink and Marshall's Nature paper attracted so much
attention is that it was seen to support a quite different theory: that
the bias is orchestrated by "higher" cortex areas.

Visual
perception seems to emerge in the brain through a hierarchical process
in which "low" areas of the brain send out signals when they detect
simple aspects of the image falling on the retina, such as vertical or
horizontal lines, or movement in different directions. These signals
are then turned into meaningful scenes by "higher" areas. But this is
not a passive process. High-level attentional areas can tell low-level
sensory areas what they should be concentrating on.

Fink
and Marshall's experiment appeared to show exactly this. Fink says that
areas around high-level regions known to be crucial for directing the
brain's attention, the inferior parietal cortex and its junction with
the temporal cortex, fired every time attention switched between local
and global features.

But very little about the brain is ever straightforward.

The team replicated the test using an "object navon" (image left), an image in which a large shape such as an anchor is
made up of smaller shapes such as cups. The pattern of activity
was utterly reversed, the scans showed left-brain activation for
processing the global picture and right-brain activation for the local
elements.

Why should using an object navon
reverse the side of the brain that is spurred into activity?

The team have yet to find an answer. Fink has a strong feeling that the wayward result is something to do
with the fact that in the object navon, the local elements are very
small, much smaller than the letters making up the letter navons. It
could be that the difficulty of discerning such small shapes changes
the nature of the task. Instead of the brain increasing the sensitivity
of the local pathway, it may be busy inhibiting awareness of the global
shape, so apparently creating a metabolic hot spot in the "wrong"
hemisphere. This, of course, is speculation and the team plans to run
more tests when they find how to match the ease of switching attention
between the local and global views of their object and letter navons.
This may mean altering the relative sizes of the elements and perhaps
using more geometrical shapes.

Overall, the bulk of the evidence still suggests that the left brain is
orchestrated to a state of local bias, while the right-side processing
is tilted towards the global. But just how these attention effects
express themselves in terms of the activity of individual brain areas
such as V2 and V3 depends rather on the nature of the task.

Sources:

'Right Brain' or 'Left Brain' - Myth Or Reality? By John McCrone "
The New Scientist" (download pdf )

The well known Hermann-grid illusion,
gives a good reflection of this shifting, balance seeking activity.
When we look at the image above, our focus shifts from the whole image
that is black and look for a point/detail at a crossing point of lines,
this activety goes back and forth, also trying to get 'visualisation',
understanding of the grid itself, who is white in contrast to the black
square. This non grip having situation starts to pop-up the blurry
points and confusion in our brain.